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Compositions for high power piezoelectric ceramicsRelated Patent Categories: Compositions, Piezoelectric, Lead, Zirconium, Titanium Or Compound Thereof ContainingCompositions for high power piezoelectric ceramics description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070181846, Compositions for high power piezoelectric ceramics. Brief Patent Description - Full Patent Description - Patent Application Claims
REFERENCE TO RELATED APPLICATIONS [0001] This application is a continuation-in-part of, and claims the benefit of, applicant's copending U.S. patent application Ser. No. 10/686,310, the entire contents of which are incorporated herein by reference. FIELD OF THE INVENTION [0002] The present invention relates generally to piezoelectric ceramic compositions, articles formed from these compositions, and methods for preparing the piezoelectric ceramic compositions and articles. BACKGROUND OF THE INVENTION [0003] Piezoelectric elements are widely used in a variety of electronic components including ceramic resonators, ceramic filters, piezoelectric displacement elements, buzzers, transducers, ultrasonic receivers and ultrasonic generators, etc. Consequently, the use of the piezoelectric ceramic compositions to form the elements is increasing. The different uses or applications require different electromechanical characteristics from the piezoelectric ceramics. Furthermore, there is continued drive towards increasingly smaller electronic components. Consequently, there is a concomitant demand for smaller piezoelectric elements to use in these electronic components. However, many of the smaller electronic components require that the piezoelectric elements provide the same or even greater output power, despite their reduced size. [0004] Existing high power piezoelectric ceramics often do not exhibit suitable electromechanical properties for use in miniaturized electronic devices, such as miniaturized ultrasonic devices. In the current state of the art, the existing piezoelectric elements that are sufficiently small to be used in the miniaturized devices exhibit low capacitance and high electrical impedance. This is inadequate to drive the miniaturized devices. Furthermore, the dielectric loss factor (tan .delta.) of the current piezoelectric elements is too high resulting in internal heating and dissipative loss, which will significantly decrease the efficiency and output of the device. Consequently, existing piezoelectric ceramics have not provided adequate electromechanical properties for these miniaturized electronic devices. [0005] The electromechanical properties of the piezoelectric ceramics can be altered by varying the specific ceramic composition, the ceramic or molecular structure, and/or the methods and parameters for fabricating the piezoelectric ceramic. [0006] Common piezoelectric ceramics can be formed from of a variety of general classes or types of ceramics. One class is a lead-zirconium titanate ceramic (PZT); another class is a lead-magnesium niobium ceramic (PMN). In many cases, solid solutions of either the PZT or the PMN ceramic are prepared in which discreet components or ceramic particles are distributed either homogeneously or inhomogeneously in the bulk matrix--either the PZT ceramic or the PMN ceramic. The discreet components can be found in the interstitial spaces of the crystal units of the bulk matrix. The added components can provide additional characteristics to the resulting piezoelectric ceramic. Additionally, dopants have been added to the piezoelectric ceramic matrix to modify a variety of factors, including the Curie temperature, the mechanical quality factor, the dielectric dissipation factor, and the like. [0007] In addition, methods used to fabricate the piezoelectric ceramics vary widely. In particular the specific heat treatment regimes and poling processes can have a dramatic effect of on both the physical properties and the electrical properties of the resulting ceramics. It is often problematic to fabricate a suitable ceramic that forms a uniform, single phase having a desired crystal structure and/or that does not crack under the strain in use. [0008] As with many ceramic fabrication techniques, the processing parameters typically are tailored to the specific ceramic composition and its intended use. Furthermore, even the known processing parameters used for a specific class or type of ceramic still can be varied to provide a unique piezoelectric ceramic having desired electromechanical properties. However, the effect of the changes on the electromechanical properties typically cannot be predicted a priori-a without extensive experimentation both as to the composition and to the method of fabrication. Consequently, it is still very difficult to adequately prepare piezoelectric ceramics having the desired electromechanical properties for miniaturized electronic devices. [0009] In light of the above problems, there is a continuing need for new piezoelectric ceramic compositions, piezoelectric elements formed from the compositions, and methods of fabricating the compositions and the elements. The present invention addresses those needs. BRIEF SUMMARY OF THE INVENTION [0010] In one form the present invention provides a novel ceramic composition having a base Formula 1 as shown below: Pb.sub.(1-z)M.sub.z(Mg.sub.1/3Nb.sub.2/3).sub.x(Zr.sub.yTi.sub.1-y).sub.1- -xO.sub.3 (1) wherein M is selected to be either Sr or Ba, x is selected to be between about 0.1 and about 0.7, y is selected to be between about 0.20 and about 0.70, and z is selected to be between about 0.02 and about 0.1. In some preferred embodiments x is selected to be between about 0.35 and about 0.40, y is selected to be between about 0.36 and about 0.42, and z is selected to be between about 0.04 and about 0.08. In other preferred embodiments x is selected to be between 0.37 and 0.38, y is selected to be between 0.38 and about 0.39, and z is selected to be between 0.05 and 0.07. [0011] In some preferred embodiments, one or more dopants are added to the composition. Examples of the dopants for use in the present invention include manganese, niobium, tellurium, molybdenum, tantalum, cobalt, and yttrium ceramics. In some preferred embodiments the dopants are be provided by the precursor group comprising: Fe.sub.2O.sub.3, CeO.sub.2, Sb.sub.2O.sub.3, Sm.sub.2O.sub.3, CuO, CdO, Ti.sub.2O.sub.3, MoO.sub.3, Nd.sub.2O.sub.3, Yb.sub.2O.sub.3, Ta.sub.2O.sub.5, Y.sub.2O.sub.3, In.sub.2O.sub.3, Tm.sub.2O.sub.3, RuO.sub.2, MnO.sub.2, Ni.sub.2O.sub.3, TeO.sub.2, MoO.sub.3, Nb.sub.2O.sub.5, and CoCO.sub.3, and mixtures thereof. [0012] The dopants can be added to the ceramic composition in individual amounts ranging up to about 5 wt. %, more preferably between about 0.01 wt % and about 2.0 wt %. In some preferred embodiments, between 0.2 and 0.4 wt. % MnO.sub.2 is used as a dopant. In some preferred embodiments, between 0.5 and 0.1.5 wt. % PbO is used as a dopant. In some preferred embodiments, between 1.0 and 2.0 wt. % Nb.sub.2O.sub.5 is used as a dopant. In one preferred embodiment, the dopant formulation comprises about 0.3 wt. % MnO.sub.2, about 1.0 wt. % PbO, and about 1.6 wt. % Nb.sub.2O.sub.5. [0013] The preferred ceramic compositions of the present invention exhibit suitable electromechanical properties for use as piezoelectric elements in miniaturized electronic devices. The preferred piezoelectric ceramics of the invention exhibit one or more of the following electromechanical properties: a relative permittivity (.epsilon.) of between about 2000 and about 4000, a mechanical quality factor (Q.sub.m) of between about 900 and about 2000; a piezoelectric strain constant (d33) of between about 250-500 PC/N, Dielectric loss factor (tan .delta.) of between about 0.002-0.008 and a thickness electromechanical coupling coefficient (k.sub.t) of between about 0.4 and about 0.7. Additionally, the preferred perovskite ceramics of the present invention have a Curie temperature value of between about 200 and about 300.degree. C. [0014] In another form the present invention provides a method for preparing a piezoelectric ceramic. The method comprises: providing a powdered mixture comprising lead, magnesium, niobium zirconium, titanium, and at least one of strontium or barium; pulverizing the powdered mixture provide a homogeneously mixture having an average particle size of less than about 1 micrometer; calcining the homogeneous mixture at a temperature of between about 800.degree. C. and about 1000.degree. C. to provide a ceramic composition; molding the ceramic composition into a green article of a desired shape; sintering the green article at a selected temperature of between about 1000.degree. C. and about 1300.degree. C. to provide a monolithic ceramic article having a perovskite crystal structure; and poling the monolithic ceramic article at a voltage selected to be between about 50 to about 80 V/mil thickness of material at a temperature of between about 100.degree. C. and about 120.degree. C. to provide the piezoelectric ceramic. [0015] It is an object of the present invention to provide high power piezoelectric ceramics. [0016] Further objects, features, aspects, forms, advantages, and benefits shall become apparent from the description and drawings contained herein. BRIEF DESCRIPTION OF THE DRAWINGS [0017] FIG. 1 is a scanned image of a micrograph of the ferroelectric ceramic composition described in Example 1 before poling. [0018] FIG. 2 is a scanned image of a micrograph of the piezoelectric ceramic produced after poling the ferroelectric ceramic composition described in Example 1. DETAILED DESCRIPTION OF THE INVENTION Continue reading about Compositions for high power piezoelectric ceramics... 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